1. Field of the Invention
The present invention relates to a method of, and a plant for, the combustion of water- and/or hydrogen-containing fuels and for recovering energy from flue gases formed during the combustion as well as for cleaning such flue gases.
During combustion of fuels which contain water and/or hydrogen, the flue gases created during the combustion contain water vapor derived from the initial water content or created when the hydrogen is oxidised. This water vapor represents a large reserve of energy because of the very high latent heat of evaporation of water.
It is known to burn water-vapor generating fuels such as oil, coal or peat with a large water content under pressure in a combustion chamber and to cool the flue gases at a maintained pressure while condensing water from the water-vapor therein. (See Swedish published patent application No. 426,982.) The latent heat of evaporation of the water is thereby recovered as condensation energy and is transferred to a coolant used to cool the flue gases. Because of the fact that the condensation of the water vapor (steam) takes place at a supra-atmospheric pressure, the energy is recovered at a higher temperature level than if the condensation had taken place at atmospheric pressure. After cooling, the flue gases are subjected, according to a known technique, to a pressure reduction in an expansion machine. The energy absorbed by the expansion machine can then be usefully employed (e.g. for driving a compressor which compresses the combustion air supplied to the combustion chamber).
2. Description of the Prior Art
The flue gases from the combustion of water-vapor generating fuels which are also sulfur-containing comprise, in addition to water vapor, sulfur trioxide among other things. During cooling to temperatures of around 400.degree. C., the sulfur trioxide combines with the water vapor to produce sulfuric acid in gaseous state. When cooling the flue gases to temperatures below the dew point of sulfuric acid, liquid sulfuric acid appears which gives rise to a very corrosive environment. During combustion of sulfur-containing water-vapor generating fuels in the manner described above, heat exchangers, conduits and chimneys of an especially acid-proof steel have been used, and the cooled exhaust gases have been maintained at as low a temperature as possible, in view of the corrosion problems described.
According to another known case, the cooled flue gases are subjected to such a rapid decrease of pressure in an expansion machine that their temperature, after having passed through the expansion machine, is sufficiently low for the impurities in the flue gases to be removed in liquid state, or in solid state, before the flue gases are discharged via a chimney. (See Swedish published patent application No. 427,691.)